1. Field of the Invention
The present invention relates to a low distortion, feed-forward microwave amplifier apparatus including an out-of-band intermodulation product suppression arrangement which enables the amplifier to operate with increased power and efficiency.
2. Description of the Related Art
Feed-forward techniques are finding increasing use in a variety of microwave communications systems. A particular application in which feed-forward amplifiers are especially useful is in multichannel amplitude modulated link (AML) transmitters for community antenna television (CATV) systems.
Typical feed-forward amplifier arrangements are designed to cancel distortion introduced by a main power amplifier. Samples of the signal being processed are obtained before and after the main amplifier, and are compared to obtain an error signal. The error signal is amplified in an auxiliary amplifier and combined with the distortion-containing signal from the main amplifier such that cancellation of the distortion components occurs. An additional distortion cancellation stage may be added when improved performance is desired. The basic principles of the feed-forward amplifier arrangement are presented in a paper entitled "A Microwave Feed-Forward Experiment", by H. Seidel, in the Bell System Technical Journal, vol. 50, no. 9, November, 1971, pp. 2879-2916.
A practical feed-forward microwave amplifier apparatus is disclosed by the present inventor in U.S. Pat. No. 4,782,307, entitled "FEED-FORWARD MICROWAVE AMPLIFIER ARRANGEMENT WITH FERRITE TEMPERATURE COMPENSATION", issued Nov. 1, 1988 and assigned to Hughes Aircraft Company, the assignee of the present invention. The basic arrangement of the amplifier is illustrated in FIG. 1. The apparatus is generally designated as 10, and includes a four port directional coupler 12 for receiving a microwave input signal IN at an input port 12b thereof. The input signal IN passes through the coupler 12 to an output port 12d thereof, and further through a variable phase shifter 14 to an input port 16b of a second directional coupler 16. The input signal IN also passes through the coupler 12 to an output port 12c thereof, and further through a variable attenuator 18 to the input of a main power amplifier 20. The coupler 12 has another input port 12a which is connected to a termination 22.
The output of the amplifier 20 is connected to an input port 16a of the directional coupler 16. The output signal from the amplifier 20 passes through the directional coupler 16 to an output port 16c thereof with negligible attenuation, and through a variable phase shifter 24 to an input port 26a of a third directional coupler 26. The output signal from the amplifier 20 also passes through the coupler 16 to an output port 16d thereof, and further through a variable attenuator 28 and an auxiliary or error amplifier 30 to an input port 26b of the coupler 26.
The apparatus 10 provides an output signal OUT with amplified power at an output port 26c of the coupler 26. The coupler 26 has another output port 26d which is connected to a termination 32. The output signal from the phase shifter 24 passes through the coupler 26 to the output port 26c with small attenuation, while the output signal from the amplifier 30 passes through the coupler 26 to the output port 26c with predetermined attenuation.
The directional couplers 12, 16 and 26 may be of conventional construction, with the desired signal attenuation determined by the coupling ratio. The gain of the feedforward amplifier is equal to the coupling ratio of the coupler 16, less the loss between 12b and 16b and the loss between 16c and 26c.
The coupling ratios of the couplers 12 and 16 are selected, and the variable attenuator 18 adjusted, such that the signal attenuation from the input port 12b of the coupler 12 to the output port 16d of the coupler 16, through the attenuator 18, amplifier 20, and coupler 16, is equal to the attenuation from the input port 12b of the coupler 12 to the output port 16d of the coupler 16, through the phase shifter 14 and coupler 16.
The coupling ratios of the couplers 16 and 26 are selected, and the variable attenuator 28 adjusted, such that the signal attenuation from the input port 16a of the coupler 16 to the output port 26c of the coupler 26 (through the attenuator 28, amplifier 30, and coupler 26) is equal to the signal attenuation from the input port 16a of the coupler 16 to the output port 26c of the coupler 26 (through the phase shifter 24 and coupler 26).
The directional coupler 12 samples the input signal IN and feeds a replica of the input signal along the output port 12d to the variable phase shifter 14, which is adjusted to cancel the signal at 16d which proceeds through 12c. The phase shifter 14 and directional coupler 16 in combination constitute a first comparator or subtractor 34 which subtracts the replica signal from the amplified version of the input signal IN which is produced or output at the output of the main power amplifier 20. More specifically, the amplified input signal and inverted replica signal are combined in the directional coupler 16. Due to the phase delay and resulting logical inversion of the replica signal, the signal combination in the coupler 16 results in the replica signal being subtracted from the amplified input signal.
Although the inverted replica signal appearing at the input port 16b of the directional coupler 16 has negligible distortion, the amplified input signal appearing at the input port 16a will include distortions introduced by the amplifier 20. The purpose of the feed-forward arrangement is to eliminate these distortions by cancellation. The amplified input signal passes through the output port 16c of the coupler 16 to the phase shifter 24 with the distortions unaffected. However, due to the equal signal attenuation in the two signal paths from the input port 12b of the coupler 12 to the output port 16d of the coupler 16 as described above, the inverted replica signal will be subtracted from the amplified input signal in the branch of the coupler 16 terminating at the output port 16d, such that the remaining signal at the output port 16d ideally consists of only the distortions.
The signal passing from the output port 16d of the coupler 16 through the attenuator 28 to the error amplifier 30 may be considered as an error signal, since it consists of the distortions or errors introduced into the input signal in the main power amplifier 20. The error signal is amplified in the amplifier 30, and applied to the input port 26b of the coupler 26.
The phase shifter 24 and directional coupler 26 in combination constitute a second comparator or subtractor 36 which subtracts the error signal from the amplified and distorted input signal. More specifically, the amplified input signal and error signal are combined in the directional coupler 26. Due to the phase delay and resulting logical inversion of the amplified input signal in the phase shifter 24, the signal combination in the coupler 26 results in the error signal being subtracted from the amplified input signal.
Microwave systems are expensive, and subject to thermal and mechanical constraints. It is therefore desirable to obtain the highest level of efficiency from each system component. Attempts to optimize the conventional feed-forward amplifier arrangement include providing the error amplifier with a lower power handling capability than the main power amplifier, which is therefore less expensive. This is possible since the amplified error signal has substantially lower power than the amplified input signal. Further attempts at optimization include driving the main power amplifier at as close to its saturation level as possible in order to maximize the output power. However, it has been found that the error amplifier introduces distortions of its own into the error signal when the main amplifier is driven to a very high level such that the associated error signal becomes very large. This effect can become the performance limitation, particularly in feedforward amplifiers utilizing dual cancellation stages.